Dear koi farmers,
As of today, the source code of Koi Farm is publicly available on GitHub.
The game is written in HTML & JavaScript and it uses WebGL to render. Programmers who are familiar with these technologies should be able to work with the code. The code is provided "as is", but Koi Farm has a lively discord with a dedicated channel for code related questions; please ask them there. For more complex issues, GitHub discussions can be used.
The code is distributed under the Apache 2.0 with Commons Clause license. That means that you are free to modify the code as you wish and distribute those modifications, but it is not allowed to sell any portions of the code. Great contributions may be included in the main game through pull requests.
I hope the code helps modders to create new content, and I also hope it's interesting to other developers. Besides that, I'm planning on writing a few articles on my blog explaining the used techniques.